Vibrating micro-electro-mechanical-system (MEMS) gyroscopes are used in a variety of systems where an angular rotation rate is to be measured. A vibrating MEMS gyroscope hereto comprises a gyroscope mass that is connected by springs to a substrate. The gyroscope mass is movable along a driving axis in resonant oscillation by the use of a drive force to provoke and maintain the movement. The drive force is supplied and controlled using a drive actuation unit and a drive measurement unit and associated circuitry.
The drive actuation unit comprises, e.g., a capacitive coupling along the driving axis between a capacitor plate on the substrate and an opposite capacitor plate on the movable gyroscope mass. A force acting on the gyroscope mass may be induced as a capacitive force by applying a voltage to the capacitor plates of the drive actuation unit, whereby the gyroscope mass is moved.
The drive measurement unit comprises, e.g., a similar pair of capacitor plates. The capacitance between the capacitor plates of the drive measurement unit is measured as a drive measurement signal and forms an indication of the displacement of the gyroscope mass along the driving axis.
A Coriolis force will apply to the gyroscope mass in the presence of an angular rotation. The Coriolis force is proportional to the velocity of the gyroscope mass, its angular rate of rotation and its mass, and perpendicular to the direction of movement. The Coriolis force hereby results in a displacement of the gyroscope mass along a sensing axis perpendicular to the driving axis. Measurement of the displacement of the gyroscope mass along the sensing axis can be used to obtain a measure of the Coriolis force and thus a measure of the angular rate of rotation. Hereto, a sense measurement unit is provided which, similar to the drive measurement unit, may comprise a capacitive coupling along the sensing axis between a sense capacitor plate on the substrate and an opposite sense capacitor plate on the movable gyroscope mass. The capacitance between the sense capacitor plates of the sense measurement unit is measured as a sense measurement signal and forms an indication of the displacement of the gyroscope mass along the sensing axis.
Determination of the angular rotation rate requires a consistent movement of the gyroscope mass along the driving axis. Hereto, the vibrating MEMS gyroscope comprises a drive circuitry to measure and control the amplitude of the, sinusoidal, movement of the gyroscope mass. In a vibrating MEMS gyroscope, measuring and controlling may be performed by determining a difference between a reference amplitude and the amplitude of the movement and controlling a gain of a drive actuation signal to control the drive actuation unit to supply the voltage to the capacitor plates of the drive actuation unit in accordance with the gain.
In known vibrating MEMS gyroscopes, the amplitude of the movement may be measured by sampling the drive measurement signal once every period at extremes of the drive measurement signal by the use of a phase-locked loop (PLL) to determine an in-phase clock with an appropriate phase relative to the drive measurement signal from drive measurement signal to coincide with the extremes of the drive measurement signal. The PLL may thus maintain the appropriate phase also when the period of the drive measurement signal is changing, e.g., due to a change of the oscillation period of the gyroscope mass due to, e.g., environmental conditions such as temperature.
Determination of the angular rotation rate is performed by determining a measure of the amplitude and relative phase of the, sinusoidal, displacement of the gyroscope mass along the sensing axis, the relative phase being measured relative to the displacement of the gyroscope mass along the driving axis. Hereto, the vibrating MEMS gyroscope comprises a sense circuitry to sample the sense measurement signal with a phase relation with the drive measurement signal using an in-phase clock to obtain in-phase sense-measurement samples and to sample the sense measurement signal at a quadrature phase relative to the drive measurement signal using a quadrature clock to obtain quadrature sense-measurement samples associated with a, so-called, quadrature signal between the sense measurement signal and the drive measurement signal. Combining the in-phase sense-measurement samples and the quadrature sense-measurement samples then allows to determine the amplitude of the displacement along the sensing axis, and thereby a measure of the angular rotation rate and a measure of an amplitude of the quadrature signal.
In known vibrating MEMS gyroscopes, a phase-locked loop (PLL) is used to establish the in-phase clock from the drive measurement signal and, in some known vibrating MEMS gyroscopes, another phase-locked loop (PLL) is used to establish the quadrature clock from the drive measurement signal.